BRØDRESKIFT, Kunt (Hennumbråtan 22, Tranby, N-3408, NO)
| P a t e n t c l a i m s 1. A system for reduction of emissions from volatile liquid cargo within a pipe work supplying cargo into a cargo tank, the pipe work including an upper pipe segment (1.1) and a lower pipe segment in the form of a mainly vertical drop pipe (1.4) terminating near the bottom of the cargo tank, and having means controlling the pressure within the pipe work during loading, c h a r a c t e r i z e d i n that a flow restricting device is installed in the upper pipe segment (1.1) in front of the drop pipe (1.4) so as to assure positive pressure throughout the upper pipe segment, and that the drop pipe (1.4) is provided with a perforated pressure equalization pipe (1.7) and a free flow dampening device (1.6) surrounding pressure equalization pipe, the upper portion of the drop pipe above the upper pipe segment (1.1) is provided with a top section (1.5) so as to form a gas collecting as well as pressure monitoring and controlling volume communicating with the pressure equalization pipe. 2. A system according to claim 1, c h a r a c t e r i z e d i n that the flow restricting device is in the form of a throttle valve (1.3) controlling upstream conditions within the upper pipe segment (1.1). 3. A system according to claim 1 and 2, c h a r a c t e r i z e d i n that the top section (1.5) is communicating with a gas compression unit (1.10). 4. A system according to any of the preceding claims, c h a r a c t e r i z e d i n that when needed the gas compression unit (1.10) is supplying compressed inert gas taken from an upper portion of the cargo tank into the top section (1.5) and pressure equalization pipe (1.7) so as to maintain pressure above the cargo bubble point pressure. 5. A system according to claim 4, c h a r a c t e r i z e d i n that inert gas is taken from the cargo tank via a pipe (1.16) and compressed inert gas is supplied into the top section (1.5) and pressure equalization pipe (1.7) via a pipe (1.17). 6. A system according to any of the preceding claims lto3, c h a r a c t e r i z e d i n that when needed the gas compression unit (1.10) is supplying compressed gas sucked from the top section (1.5) into liquid cargo within the cargo tank or used as fuel for a thermal combustion unit (1.22). 7. A system according to claim 6, c h a r a c t e r i z e d i n that compressed gas is supplied into the cargo tank via a pipe (1.18) provided with an absorber (1.21), the absorber being situated at the bottom of tank, submerge in liquid cargo. 8. A system according to any of the preceding claims, c h a r a c t e r i z e d i n that the free flow dampening device is in the form of a continuous or segmented helical flow surface (1.6). 9. A system according to claim 6, c h a r a c t e r i z e d i n that a flow stabilizer (1.18) is provided at the outlet of the drop pipe (1.4). 10. A system according to any of the preceding claims, c h a r a c t e r i z e d i n that cargo level in the drop pipe is monitored with signal from two pressure transmitters (1.11, 1.12), one at the bottom and the top, respectively. |
The invention relates to a system for reduction of emissions from volatile liquid cargo when loading cargo tanks, for instance, and in particular to avoid a conventional venting of hydrocarbon gas released both in the pipe work and cargo tanks into the atmosphere causing environmental problems and economical loss. Thus, some typical applications are for crude oil tankers, floating storage units, and landbased vented tanks in which a supply pipe work directs such volatile liquid cargo into the cargo tanks from top.
Vapours are initially released within the pipe work when pipe segments are routed at high altitude levels towards the cargo tank receiving cargo. As the vessels sometimes are outfitted with loading arms, or pipes segments are routed over the bow, it is often more than 35 m from the highest point of the pipe work down to the tank level. Provided free flow towards the cargo tanks, the result is obviously the occurrence of a pressure drop. Thereby, it is created a vacuum below the true vapour pressure (bubble point) of the cargo and light fractions of gas is flashing off. Such gas fractions are brought with the liquid cargo towards the cargo tanks and may to some extent be reabsorbed when exposed to higher pressure on its way to the tank. However, some gas escape and are released in the cargo tanks. The gas emissions have to be handled as the tanks get filled, e.g. vented to the atmosphere.
There exist systems that are constructed to reduce the release of gas during loading of volatile liquid cargo. One solution is presented by US-B 2 7,597,115 involving the use of a vertically oriented drop pipe or vessel within the cargo tank having big diameter which are complicated and costly to install, and there are no means to assure the conditions within the pipe work upstream the drop pipe or vessel.
Thus, it is a need of solutions eliminating such disadvantages and providing for improved performance.
SUMMARY OF THE INVENTION
This need is complied with by means of a system for reduction of emissions from volatile liquid cargo within a pipe work supplying cargo into a cargo tank, the pipe work including an upper pipe segment and a lower pipe segment in the form of a mainly vertical drop pipe terminating near the bottom of the cargo tank, and having means controlling the pressure within the pipe work during loading, wherein a flow restricting device is installed in the upper pipe segment in front of the drop pipe so as to assure positive pressure throughout the upper pipe segment, and wherein the drop pipe is provided with a perforated pressure equalization pipe and a free flow dampening device surrounding pressure equalization pipe, the upper portion of the drop pipe above the upper pipe segment is provided with a top section so as to form a gas collecting as well as pressure monitoring and controlling volume communicating with the pressure equalization pipe.
The flow restricting device can preferentially be in the form of a throttle valve controlling upstream conditions within the upper pipe segment.
To actively control pressure within the drop pipe, the top section is communicating with a gas compression unit. When needed to maintain pressure above the cargo bubble point pressure, the gas compression unit can supply compressed inert gas taken from an upper portion of the cargo tank into the top section and pressure equalization pipe.
Further, when needed to control pressure additionally, the gas compression unit is supplying compressed gas sucked from the top section into liquid cargo within the cargo tank or used as fuel for a thermal combustion unit. In such instances, the compressed gas can be supplied into the cargo tank via an absorber being situated at the bottom of tank, submerge in liquid cargo.
The free flow dampening device is preferentially in the form of a continuous or segmented helical flow surface, while a flow stabilizer is provided at the outlet of the drop Pipe- Thereby, it is provided a system mainly eliminating the release of volatile hydrocarbons within the supply pipe work for to cargo tanks. The system is also able to stabilize the cargo before it enters the tanks so as to minimise further emissions inside the cargo tanks. Existing load supply pipe elements towards the tanks can be used as part of the invention.
When cargo enters the area on top of the cargo tanks, it is often present a high point somewhere along the upper pipe segment. Such a high point is causing -specially in the beginning of loading- static pressure differences between the cargo tank and high point in the pipe work and the occurrence of flashing. To avoid this situation, a specially developed throttle valve is mounted before the pipe work drops vertically into the cargo tank. The valve maintains a positive pressure upstream and prevents release of gas in the pipe work at the high points. The valve is specially designed to perform a throttling by reducing the pressure gradually without flashing. Further, the drop pipe is outfitted with a free flow restrictor and pressure equalization pipe. By damping the speed of flow downwards, generation of vacuum and release of gas is avoided. The perforated pipe centred in the drop pipe equalizes the pressure so as to preventing that a local vacuum is generated.
The gas pressure in the top section of the drop pipe is actively used in controlling emissions as an increase of pressure reduces or eliminates flashing, whereas a reduction of pressure increases flashing so as to stabilize the cargo. Pressure control is affected using the gas compression unit (GCU). The GCU feeds the gas to an absorber in the cargo tanks or to a gas consumer such as a thermal combustion machine for use as pure fuel. If pressure need to be increased, the GCU suck gas from cargo tanks and supply gas to the top of the drop pipe so as to build up pressure. The cargo is further fed by gravity but restricted in acceleration and fed smoothly into the cargo tanks, with no further release of cargo vapours.
DETAILED DISCUSSION OF THE INVENTION
The present invention is now to be discussed in more detail with reference to the accompanying drawing, in which:
Fig. 1 is a schematical view of the present system; and
Fig. 2 is schematical views of a throttle valve for use in the present system represented in cross-section and side elevation, respectively.
As shown in Fig. 1, the loading pipe work includes an upper pipe segment 1.1. A high point 1.2 on the loading pipe work can be formed by special loading arms or from pipes routed over a bow or stern of the vessel. A throttle valve 1.3 is mounted in the upper pipe segment on top of a cargo tank 1.20 before a drop pipe 1.4 enters the cargo area 1.20. The cargo area can advantageously be subdivided in at least two separate volumes.
The throttle valvel .3 is designed to give a gradual pressure drop in order to avoid local flashing through the valve. The throttle valve controls the upstream conditions to assure positive pressure at the high point 1.2. As illustrated in Fig. 2, the throttle valve is preferentially in the form of a two-stage valve outfitted with two walls 1.31, 1.32 of a certain thickness perforated with a number of nozzles having orifices 1.33, 1.34. Due to their length, the nozzles have a defined and gradual pressure loss caused by friction in the orifices. The orifices are working in parallel. The numbers opened by a gate 1.35 defines the capacity. The gate is operated using a linear actuator 1.36. This gives an overall linear flow rate as a function of the gate position minimizing a local pressure drop compared with a single throttling point across a short distance. The result is local high liquid speed and consequently low static pressure.
After the throttle valve, the cargo enters a lower pipe segment in the form of a mainly vertical drop pipe 1.4. feeding cargo into the cargo tank via distribution pipes 1.19. Due to the height difference from entrance to outlet, and cargo tank valves normally being open, gravity forces accelerate the cargo resulting in a low pressure and occurrence of flashing. To avoid such flashing, the drop pipe is provided with a top section 1.5 for gas collection as well as pressure monitoring and control. A free flow dampening device 1.6 ensures a smooth flow downwards in the drop pipe towards a liquid surface 1.4 with a gas atmosphere on top. The free flow dampening device is preferentially in the form of a continuous or segmented helical flow surface. The drop pipe functions as a big separator. The gas atmosphere in the top is to be pressure controlled, based on two criteria:
- elimination of flashing by keeping the pressure well above cargo bubble point pressure which is accomplished by using a gas compression unit 1.10 to feed gas into the top of the drop pipe and to suck inert gas from the cargo tank.
- stabilizing the volatile liquid cargo, e.g. crude, by reducing the pressure to or below volatile liquid cargo bubble point.
Thus, by sucking gas from the drop pipe 1.4 and top section 1.5 with the GCU 1.10, a controlled flashing is achieved, The gas is then returned to cargo tank via pipe 1.18 and the absorber 1.21 situated below liquid level, or used as fuel, e.g. pure hydrocarbon gas at 2-3 bar, to a thermal combustion unit 1.22, typically a boiler or a gas engine, via a pipe 1.19.
The GCU 1.10 can be constructed in any appropriate manner, and is here presented in an embodiment including a knock out drum 1.101, a compressor 1.102 of positive displacement type, and a cooler 1.103 provided with an air fan 1.104, or alternatively water cooled. The emitted vapour is fed into the GCU via a pipe 1.15. When needed inert gas from the cargo tank is fed into the GCU via a pipe 1.16 and resulting compressed inert gas is supplied into the top section via a pipe 1.17. The knock out drum 1.101 functions like a separator removing possible drops and particles from the emitted vapour as to facilitate further processing.
The absorber 1.21 is favourably of the type presented by NO 20093784 having a lower ejector, an intermediate mixer, and an upper distributor. The vapour may be fed from below or sideways inclined through the ejector and up into the mixer and distributor. As liquid cargo is flowing into the absorber propelled by the absorber itself, the vertical design especially of mixer and distributor and position is important. Further, the ejector is formed with a throttle and diffuser section having a narrowed cross-section, and a vapour inlet surrounded by a liquid inlet. In any appropriate manner the mixer is adapted to mechanically mix the vapour and liquid cargo flowing from the ejector. The distributor is formed with a number of cones arranged to surround one another and orientated in a direction outwardly inclined to the longitudinal of the absorber. The main purpose of the distributor is to expose the surface of crude against any remaining vapours to get a final absorption of vapours. Moreover, the cross-section between adjacent cones is optimized to facilitate optimal flow rate at exit in the upstream direction. The distributor has a cargo inlet arranged at the transition from the mixer. To prevent escape of any vapour from the absorber, the cones are formed with protrusion mixing the flow of liquid cargo and remaining vapour for a final absorption. The protrusions are spaced in mutual distance from one another on the respective side of each cone, and are preferentially extending peripheral around the cone surfaces.
Further, the drop pipe 1.4 has a pressure equalization pipe 1.7 in the centre. The pressure equalization pipe is perforated and open to the upper area of the drop pipe 1.4 so as to ensure pressure equalization therein, without danger for flashing. At the bottom of the drop pipe has an appropriate flow stabilizer 1.8 so as to assure smooth flow entrance into the distribution pipe segments 1.19 towards the cargo tank volumes 1.20.
To control the situation in the drop pipe, the level is monitored with pressure transmitters 1.11, 1.12 at low and high levels, respectively. In addition a level switch 1.13 is mounted at the top to safeguard that liquid is not sucked out of the pipe if the gas compression unit is in operation.
Control of the pressure in the upper part of the cargo drop pipe is accomplished with the GCU 1.10 supplying gas from the cargo tanks and, thus, maintaining sufficient pressure in the upper portion of the drop pipe 1.4 to eliminate flashing, or if for some reason the pressure is increased above preset levels gas is extracted and returned to cargo via the absorber 1.21. Cargo can be stabilized in the drop pipe before entering the cargo tank. Such stabilization can be achieved by adjusting gas pressure in the drop pipe in a manner providing for controlled flashing by means of GCU sucking and returning gas from and back to the cargo tank via the absorber or gas is supplied as fuel to a thermal en- gine, 1.22.
Although only one cargo tank and drop pipe are represented in the drawings, it is understood that any combinations of such cargo tanks and drop pipes are possible, ranging from one drop pipe for all tanks to one drop pipe for each tank.
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